Optical amplifiers can be implemented by providing a gain medium in the optical path of a light signal, for example, by forming an optical waveguide on a substrate. A gain medium typically requires an inverted population, in which more atoms or molecules are in an excited state than are in a state having less energy. A semiconductor junction can provide the electrical pump energy needed to obtain an inverted population in a gain medium. Optical pumping can also be used.
The term “laser” refers to the amplification of light by the stimulated emission of radiation. Lasers include a gain medium and a feedback mechanism. Energy is added to the gain medium to induce a population inversion, wherein the lasing species has more electrons in an outer shell (or high-energy state) than in an inner shell (or low-energy state). Interaction from a passing photon with a species having an inverted state can induce stimulated emission, wherein two photons are output, each having the same wavelength, phase, and polarization as the input photon. This gain in the number of photons provides the amplification needed by the laser.
In a laser, an active material, for example, a semiconductor, or a glass suitably doped with an active atomic species such as neodymium, is placed in a cavity resonator with a feedback path formed by, e.g., two reflecting or at least partially reflecting mirrors.
The laser will predominately output light having the characteristics (modes) that are amplified the most. These modes result from an interaction of the optical feedback and filtering with the inverted population (the characteristics of the source of energy). The modes include such characteristics as the spatial shape and spreading of the laser beam, its polarization(s), wavelength(s), etc. Since there will be a fixed amount of input energy available in the laser, once one mode is amplified even slightly more than another mode, that one mode will take more of the available energy (becoming the dominant mode), leaving less energy for the other modes. Various parts of the laser beam (e.g., across its cross section) can lase in different modes. Modes can also vary over time.
In a gain medium of a laser or an amplifier, the interaction of light in the waveguide and the surface of the substrate can differentially filter orthogonally polarized components of the laser beam. That is, one linear polarization mode can be suppressed, while the orthogonal polarization mode may not be. Thus, solid-state lasers having waveguides formed at the surface of a substrate will tend to lase with polarized modes, at least to some extent. Amplifiers having waveguides formed at the surface of a substrate will tend to amplify one polarized mode more than another, at least to some extent.
There is thus a need to provide a compensation mechanism that counteracts or interacts with the effect of waveguide polarization, in order to obtain a desired overall polarization function (such as no differential gain between polarization modes).